Printed circuit board for mounting at least one electronic part

ABSTRACT

A printed circuit board is provided to which a device such as a tape carrier package having a multiplicity of leads arranged with a small pitch is connected by a local heating method using a bonding tool. The upper surfaces of solder layers provided on lands formed on the printed circuit board are flattened prior to leads of an electronic part to be mounted being placed on the solder layers. The leads are soldered to the lands by being pressed and heated with the bonding tool. At least a region of the printed circuit board corresponding to the bottom surface of the bonding tool has a height lower than that of the leads superposed on the lands, so that the desired parallelism of the bottom surface of the bonding tool with respect to the printed circuit board is maintained.

This is a Continuation of application Ser. No. 08/438,031 filed May 8,1995 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printed circuit board havingelectronic parts mounted at a high density and a method of manufacturingthe same. More particularly, the present invention relates to astructure of a printed circuit board and a method of mounting integratedcircuit (IC) packages, a tape automated bonding (TAB) type of tapecarrier package (TCP) and the like on the printing circuit board in asurface mounting manner.

2. Description of Related Art

Conventionally, soldering has widely been used to mount electronic partson printed circuit boards. In the field of mounting by soldering, withthe advancement of miniaturization and combination of parts based on thedevelopment of large-scale semiconductor IC integration techniques, themethod of mounting package type and bare chip type IC parts in a surfacemounting manner has come into major use replacing the early-developedmethod of mounting parts with leads such as single-in-line packages(SIP) and dual-in-line packages (DIP) by inserting the leads in acircuit board. Such surface mounting is advantageous in terms ofassembly automatization and electronic characteristics. In particular,TCPs have a large chip area and are suitable for a multi-pinarrangement. TCPs also have the advantage of being mountable at a highdensity because the lead pitch of the TCPs can be reduced.

Surface mount devices (SMD) are usually connected to circuit boards byreflow soldering. Reflow soldering is performed in such a manner that aSMD is set on solder layers previously formed on electrode pads (lands)on a circuit board, and the solder layers are melted by being heated ina reflow furnace to fix the SMD to the printed circuit board. On theother hand, TCPs are ordinarily connected in such a manner that outerleads of a TCP are pressed with a bonding tool against solder layerspreviously formed on lands on a circuit board, and the solder layerand/or outer leads are heated by a local heating device while beingpressed against each other. This is because the outer leads of TCPs arevery soft and deformable. Such solder layers are formed, for example, byprinting cream solder on the circuit board where no parts are mounted.

To improve the wettability of solder, flux is ordinarily used. Recently,from the viewpoint of earth environmental protection, use ofwater-soluble or cleaning-free type flux for soldering has beenincreased. Alternatively, alloying by thermocompression bonding, amethod of using an anisotropic conductive sheet or a photo-curableinsulating resin and other methods are used instead of soldering tomount the electronic parts.

According to the above-described conventional mounting methods, if oneor more SMDs and TCPs are mounted on one circuit board, a process stepfor connecting the TCP is ordinarily performed after a reflow step forconnecting the SMD. In such a case, as shown in FIG. 12, solder layers31 located on lands 35 to which a TCP are to be connected are melted bythe preceding reflow step so as to round into a semispherical shape bythe effect of surface tension. The outer leads 32 of the TCP placed onthe solder layers 31 may slip down leftward or rightward as shown inFIG. 12 when pressed by a bonding tool 33, so that the outer leads 32are not accurately bonded to the corresponding lands 35, resulting inoccurrence of connection failure, i.e., short-circuit or the like, evenif the pressing direction is perpendicular to circuit board 34.Therefore, there is a problem of a reduction in yield and, hence, anincrease in manufacturing cost. Positioning of the outer leads of a TCPon a circuit board becomes more difficult and a misalignment of theleads can occur more easily if the pitch between the outer leads becomessmaller with the increase in the integration density or the density ofICs. If a special positioning means or fixing means is used to cope withthis problem, then a troublesome operation is required to use suchmeans; the amount of labor and time taken for positioning is increasedand the manufacturing cost is also increased.

The development of SMDs having a relatively large number of pins and arelatively small pin pitch have also been promoted. In the case ofsimultaneously mounting a multiplicity of SMDs on a circuit board in asurface mounting manner, it is difficult to accurately position theleads of the devices, and there is a substantially high probability ofoccurrence of solder bridging and misalignment. Further, if SMDs andTCPs are both mounted on a circuit board, the total number of mountingsteps is increased and the time required for mounting is relativelylong, because the SMDs and TCPs are mounted by separate connectingsteps.

Leads of electronic parts may be soldered to lands on a printed circuitboard, for example, in a manner described below. First, as shown in FIG.13A, a dispenser 36 is used to supply flux 37 to a region including aland 34 on the printed circuit board. If the flux is a water-soluble orcleaning-free type flux as mentioned above, it may spread only to suchan extent as to partially cover the surface of a resist layersurrounding the land 34, since the viscosity of the flux is low.Therefore, there is a possibility of failure to sufficiently immerse aconnected end portion of a lead 32 placed on a solder layer 31. If theconnected end portion of lead 32 is pressed and heated by a bonding tool39, as shown in FIG. 13B when not sufficiently immersed in the flux 37,the connected end portion of lead 32 will not be sufficiently wettedwith the solder. In such a situation, the probability of occurrence ofconnection failure is high and the yield is reduced. In particular, inthe case of a part such as a TCP having a multiplicity of leads arrangedwith a very small pitch, a troublesome correction operation and a longtime are required to correct even a connection failure of only one lead.

When leads and lands a pressed against each other with a bonding tool asdescribed above, it is important to maintain a certain parallelismbetween the bottom surface of the bonding tool and the surface of theprinting circuit board. As shown in FIG. 14, if a bottom surface 40 of abonding tool 39 presses leads 32 of electronic parts in a state of beinginclined with respect to a circuit board 34, there is a possibility ofthe leads slipping down solder layers 31 of corresponding lands 35causing connection failures or short-circuits in the wiring.

If the lead pitch is very small, as in the case of a TCP, a misalignmentcan occur easily between the leads and lands even if the inclination ofthe bonding tool is small. For example, in a case where a through holeis formed in the vicinity of a land or a material is attached to anapplied layer of a resist 38 on a wiring pattern 41 by silk screenprinting or the like so as to form a character 42 or the like, thebottom surface 40 of the bonding tool may abut one-sidedly against theprotrusion of the character 42 or the like so that the desiredparallelism is lost if the height of the character or the like is largerthan that of the lands by an amount such as 20 mm. If a multiplicity ofleads 32 and corresponding lands 35 are arranged with a small pitch, thebonding tool may be replaced by a different bonding tool having a bottomsurface size and a bottom surface configuration conforming to the leadarrangement to avoid the above-described problem of parallelism. In sucha case, however, a universality of the bonding tool is lost; a need forchanging and positioning a bonding tool for each process step arises aswell as a need for preparing various types of bonding tools. A largeamount of labor and time are thereby required, resulting in aconsiderable increase in manufacturing costs.

On a printed circuit board, a plurality of marks for accuratelypositioning electronic parts which are to be connected to the printedcircuit board may be provided. Such positioning marks are sometimesformed by portions of wiring patterns, but are ordinarily provided on acircuit board as marks independent of wiring patterns. Japanese PatentLaid-Open Publication No. 60-161693 discloses a printed circuit boardarranged with such recognition marks provided at arbitrary positions onthe board, particularly in the vicinity of mount portions of electronicparts so that the electronic parts can be accurately positioned.However, if such positioning marks are provided only at arbitrarypositions, a need for increasing the area of the printed circuit boardarises and it is difficult to increase the density of mounted parts andthe density of wiring. Moreover, the circuit wiring becomes complicatedand the freedom of circuit design is reduced, so that the circuit designcannot be sufficiently adapted for miniaturization of the circuit boardand semiconductor devices. It is also necessary for the positioningmarks to be formed so as to be clearly recognizable, because thepositioning marks are optically detected by a sensor.

SUMMARY OF THE INVENTION

In view of the above-described problems of the conventional art, anobject of the present invention is to provide a printed circuit boardwhich is designed to cope with the problem of a reduction in lead pitchand an increase in density of ICs, and on which electronic parts such asan SMD and a TCP are soldered can be positioned accurately and can bemounted in a surface mounting manner so as to be maintained in a goodconnected condition.

Another object of the present invention is to provide a method whichensures that electronic parts such as an SMD and a TCP to be mounted ona printed circuit board can be positioned accurately and can be solderedeasily at a low cost and maintained in a good connected condition.Specifically, the present invention aims to provide a method suitablefor mounting a TCP or SMD on a printed circuit board withoutsignificantly changing the conventional mounting process, withoutsignificantly increasing the number of steps of the conventionalmounting process and without using any special position and fixing meansbut still ensuring that the above-described objects are achieved.

Still another object of the present invention is to provide a method forsimultaneously mounting a multiplicity of electronic parts in a surfacemounting manner so that the electronic parts are maintained in a goodconnected condition by soldering.

Yet another object of the present invention is to provide a printedcircuit board arranged to enable flux to supplied by a comparativelysimple means so that leads of electronic parts are sufficiently immersedin the flux when leads of electronic parts are soldered by local heatingusing a bonding tool, whereby the wettability of the solder is improvedto enable each lead of the electronic parts to be connected in a goodcondition.

Still another object of the present invention is to provide anelectronic parts connection method which makes it possible tosufficiently supply flux in a comparatively simple manner so that leadsof electronic parts may be suitably connected to lands on a circuitboard.

A further object of the present invention is to provide a printedcircuit board arranged so that while a certain universality of a bondingtool is maintained, the bonding tool is set with a suitable parallelismto limit misalignments of leads of electronic parts on lands to whichthe leads are to be connected, whereby the leads can always be connectedin a good condition.

Still a further object of the present invention is to provide a printedcircuit board arranged so that positioning marks more clearlyrecognizable are provided by effectively utilizing the area of thecircuit board to improve the degree of freedom of circuit design, andwhich can be suitably reduced in size while the density of parts mountedthereon is increased.

To achieve the above-described objects, according to one aspect of thepresent invention, there is provided a printed circuit board whichincludes lands to which leads of at least one electronic part to bemounted are connected, and a solder layer formed on each of the lands,wherein the upper surface of the solder layer is flattened.

In the above-described printed circuit board, the solder layers havingthe flat upper surface and the lands may be covered with a removableheat resistant resin material before the leads of the electronic partare connected thereto.

According to another aspect of the present invention, there is provideda method of forming a printed circuit board having lands correspondingto leads of at least one electronic part to be mounted with a solderlayer formed on each land. The upper surface of the solder layer isflattened prior to mounting the at least one electronic part. The methodfurther comprises placing leads of the electronic part on the solderlayer, and connecting the lands and the leads by soldering.

In the above-described method, the upper surface of the solder layer maybe flattened by pressing. Pressing may be performed by using at leastone roller. Further, the length of the roller may be longer than thelength of one side of the printed circuit board, and the entire surfaceof the printed circuit board may be pressed simultaneously.Alternatively, the printed circuit board may be partially pressed byusing the roller.

In the above-described method, pressing may alternatively be performedby using a bonding tool. Further, a plurality of the printed circuitboards may be stacked one on another and may be pressed simultaneouslyby using the bonding tool.

In the above-described method, pressing may alternatively be performedby pinching the printed circuit board between metallic plates. Further,a plurality of printed circuit boards may be stacked one on another andmay be pressed by using the metallic plates. The entire surface of theprinted circuit board may be pressed simultaneously by using themetallic plates. Alternatively, the printed circuit board may bepartially pressed by using the metallic plates.

In the above-described method, the leads placed on the solder layers andthe lands may be soldered by being heated. Leads of a plurality ofelectronic parts may be placed on the corresponding lands and may besoldered simultaneously. The plurality of electronic parts may include atape carrier package type part. The leads and the lands may be solderedby being pressed simultaneously with the heating.

In the above-described method, the electronic part may be a tape carrierpackage type part and may be connected after a reflow step.

Further, the above-described method may be such that, after the uppersurfaces of the solder layers have been flattened, the solder layers andthe lands are temporarily covered with a heat resistant resin material,the resin material being thereafter removed, and the leads of theelectronic part then being placed on the solder layer.

According to yet another aspect of the present invention, there isprovided a printed circuit board comprising a plurality of lands forconnection to leads of at least one electronic part to be mounted, asolder layer formed on each of the plurality of lands, and at least oneframe formed so as to surround the lands and so as to be higher than thesolder layer.

In this printed circuit board, the frame may be formed by silk screenprinting.

According to still another aspect of the present invention, there isprovided a method of forming the above-described printed circuit board,comprising supplying flux to a region surrounded by the frame, placingthe leads of the electronic part on the solder layers so that the leadsare immersed at least partially in the flux, and pressing and heatingthe leads, the solder layers and the lands to connect the lands and theleads.

According to a further aspect of the present invention, there isprovided a printed circuit board comprising a plurality lands forconnection to leads of at least one electronic part to be mounted bybeing pressed with a bonding tool, and a region on the printed circuitboard lower in height than the lands and provided in an areacorresponding to a bottom surface of the bonding tool.

In this printed circuit board, the at least one electronic part may haveleads arranged along its four sides, and the region may be provided in arectangular frame-like area which corresponds to the leads and in whichthe lands are located.

According to still a further aspect of the present invention, there isprovided a printed circuit board comprising a substrate on which atleast one electronic part chip is mounted, and positioning marks forpositioning the at least one electronic part chip when the chip ismounted, wherein the positioning marks are provided inside a region ofthe substrate to which the electronic part chip is directly fixed.

According to still a further aspect of the present invention, there isprovided a printed circuit board comprising a substrate on which atleast one electronic part chip is mounted, and positioning marks forpositioning the at least one electronic part chip when the chip ismounted, wherein the substrate has a multilayer structure having aninner layer processed by a blackening treatment at positionscorresponding to the positioning marks.

In the printed circuit board in the first aspect of the presentinvention, when the leads of the electronic part to be mounted areplaced on the solder layers, the leads are prevented from slipping down,so that the leads and the lands can be stably maintained in a goodconnected condition.

In addition, when a heat treatment such as a reflow step is previouslyperformed to other connected electronic parts, the desired flatness ofthe upper surfaces of the solder layers used in a subsequent step can bemaintained.

In the electronic part connection method in the second aspect of thepresent invention, when the leads of the electronic part are placed onthe upper surfaces of the solder layers, there is substantially nopossibility of the leads slipping down, and the leads can be soldered ina state of being correctly positioned. It is therefore possible tosuitably connect the leads and lands reliably in a very simple manner ifonly a few process steps are added to flatten the solder layers.

In the electronic part connection method, the conventional mountingprocess can be utilized without being changed except that a flatteningstep is added before the step of placing the leads of the electronicpart, whereby the solder layers can be easily flattened. This flatteningcan be performed by pressing in a simple manner without largely chargingthe conventional process, and the pressing step may be a step of rollinga roller on the upper surfaces of the solder layers, a step utilizing abonding tool or a step of pinching the printed circuit board betweenmetallic plates. Also a plurality of printed circuit boards may bestacked and pressed to flatten the solder layers simultaneously.Further, the solder layers may be partially melted by heating toflattened more easily.

In the electronic part connection method in the second aspect of theinvention, the solder layers on which the leads of the at least oneelectronic part are placed are melted by heating to perform reflowsoldering. A plurality of electronic parts can also be soldered atsimultaneously by one step to be maintained in a good connectedcondition. Also, an SMD and a TCP can be simultaneously mounted in asurface mounting manner by one reflow step. It is also possible toconnect the leads of the electronic part by local heating without anymisalignment between the leads and the lands.

In a case where a TCP is connected after the reflow step, the TCP havinga multiplicity of pins and a small lead pitch can be suitably connectedto the printed circuit board without being affected by heating of thereflow step.

Even if heat is applied to the printed circuit board by the reflow stepto other connected electronic parts after flattening the upper surfacesof the solder layers, the desired flatness of the upper surfaces of thesolder layers can be maintained before a resin material covering thecircuit board is removed.

In the printed circuit board in the third aspect of the presentinvention, even if a water-soluble or cleaning-free type flux having acomparatively low viscosity is used, the flow of flux is limited by theframe so that a sufficiently large amount of flux can be supplied to aregion including the lands, thereby enabling the leads to besufficiently immersed in the flux.

On this printed circuit board, a frame can be formed simultaneously withvarious characters or symbols provided on the surface of the printedcircuit board by silk screen printing.

In the electronic part connection method in the fourth aspect of thepresent invention, a sufficiently large amount of flux can be suppliedto connected portions of the lands and the leads by the effect of theframe, so that the solder can suitably wet the leads to improve thesoldering effect.

In the printed circuit board in the fifth aspect of the presentinvention, when the lands and the leads placed on the lands are pressedby using a bonding tool, the bottom surface of the bonding tool does notcontact a surface portion of the circuit board other than the leads, sothat the desired parallelism of the bottom surface with respect to thecircuit board surface can be maintained.

In this printed circuit board, even in a case where all the leads areprovided along the four sides of the electronic part and are connectedto the corresponding lands by performing pressing once with a bondingtool, the desired parallelism of the bottom surface of the bonding toolcan be maintained.

In the printed circuit board in the sixth aspect of the presentinvention, an area inside the area on the substrate where an electronicpart chip is directly connected is utilized for releasing heat from thechip and/or ground line connection and no signal conductors are providedin this area. Accordingly, positioning marks can be provided in thisarea without any hindrance to circuit design on the substrate. Also, themarks can be formed independently of wiring patterns on the substrate tohave improved recognizability.

In the printed circuit board according to the seventh aspect of thepresent invention, positioning marks can be recognized more clearly bythe effect of contrast with an inner layer processed by a blackeningtreatment.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments of the invention with reference to the accompanying drawingswhere like reference numerals refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a printed circuit board unitto which the present invention is applied;

FIG. 2 is a schematic perspective view of a printed circuit board foruse in the printed circuit board unit shown in FIG. 1;

FIGS. 3A and 3B are cross-sectional views of a method of flatteningsolder layers formed on lands on the printed circuit board;

FIG. 4 is a cross-sectional view of another method of flattening solderlayers;

FIG. 5 is a cross-sectional view of still another method of flatteningsolder layers;

FIG. 6 is a plan view of lands and frames formed on the printed circuitboard of the present invention;

FIG. 7 is a cross-sectional view taken along the line VII--VII of FIG.6;

FIGS. 8A and 8B are enlarged cross-sectional views of process steps forsoldering outer leads to the lands shown in FIG. 7 using flux;

FIG. 9 is a plan view showing positioning marks formed on the printedcircuit board of the present invention;

FIG. 10 is a cross-sectional view taken along the line X--X of FIG. 9showing a state of outer leads and lands connected by using a bondingtool;

FIG. 11 is a plan view of another example of the arrangement shown inFIG. 10;

FIG. 12 is a cross-sectional view of a conventional printed circuitboard when outer leads of a TCP and lands are connected to each other;

FIGS. 13A and 13B are enlarged cross-sectional views of process stepsfor soldering the outer leads to the lands on the conventional printedcircuit board by using flux; and

FIG. 14 is a cross-sectional view of the conventional printed circuitboard when the outer leads and the lands are connected to each other byusing a bonding tool.

FIG. 15 is a cross-sectional view taken along the line XV--XV of FIG. 9showing the inner layer formed by a blackening treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an embodiment of a printed circuitboard unit 1 to which the present invention is applied. The printedcircuit board unit 1 is used as a card-type computer. The printedcircuit board unit 1 may be constructed in such a manner that a TCPformed of a CPU for the computer, a sub board 4, on which electronicparts including, for example, an electrically programmable read onlymemory (EPROM) are mounted, and other electronic parts (not shown) aremounted on a printed circuit board 2 in accordance with the presentinvention. The printed circuit board unit 1 may be a double-side mounttype circuit board having a multilayer structure. A TCP, a sub board andother electronic parts similar to those mentioned above are also mountedon the reverse surface (not shown) of the printed circuit board unit 1.A connection terminal array 5 for connection to a memory board forming amain memory for the above-mentioned computer is formed in the vicinityof the TCP 3 along one of two major sides of the printed circuit boardunit 1, while a connector 6 is provided along the other major sides ofthe printed circuit board unit 1 and may extend through the entirelength of the same.

FIG. 2 illustrates the printed circuit board 2 before the TCP 3, the subboard 4 and other electronic parts are mounted. Lands 8 are formed onthe surface of the printed circuit board 2 to which outer leads 7disposed with a small pitch on four sides of the TCP 3 are to beconnected. The lands 8 are formed at positions corresponding to theouter leads 7. Also, a multiplicity of lands 10 are formed on thesurface of the printed circuit board 2 to which terminals 9 provided onperipheral portions of the sub board 4 are to be connected. The lands 10are formed at positions corresponding to the terminals 9. In thisembodiment, the sub board 4 is connected by reflow solderingsimultaneously with other surface-mounted parts (not shown). The TCP 3is connected by a well-known local heating method of pressing andheating only connected portions with a bonding tool, as described above,after the sub board 4 and other parts have been soldered by a reflowstep and after the other electronic parts have been soldered by a flowstep, i.e., at the end of a mounting process.

As shown in FIG. 3A, lands 8 are arranged on the printed circuit board 2with a small pitch, and a solder layer 11 is formed on each land 8.Solder layer 11 is previously formed, for example, by melting amultiplicity of small solder grains bonded to each land surface alongwith other soldering connection portions on the printed circuit board 2before the operation of mounting the parts is started. Therefore, solderlayer 11 is solidified while being formed into a semispherical shape byits surface tension, as shown in FIG. 3A, before being melted by theabove-mentioned reflow step for connecting the sub board 4 and otherparts.

According to the present invention, as shown in FIG. 3B, the uppersurface of each solder layer 11 is pressed into a flat shape. This maybe accomplished by, for example, a roller 12 rolled on the printedcircuit board 2 and moved in a horizontal direction along the same toapply a certain load upon the upper surface of each round solder layer11, thereby pressing the same into a flat shape.

While roller 12 is rolled on the printed circuit board 2 in theabove-described embodiment, pressing may be performed by moving theprinted circuit board 2 while maintaining the roller 12 at apredetermined position so that the upper surfaces of each solder layer11 are pressed into a flat shape by the roller 12 according to anotherembodiment of the present invention. In this case, the printed circuitboard 2 may be moved along one roller 12 or the printed circuit board 2may be passed through a nip between a pair of parallel to flatten theupper surface of each solder layer 11.

When the outer leads 7 of the TCP 3 are placed on the upper surface ofthe solder layers 11 flattened as described above according to thepresent invention, the outer leads 7 do not slip down as may happen onthe conventional printed circuit board shown in FIG. 12. Thus, the outerleads 7 of the TCP3 can be stably soldered and suitably connected. Theabove-described operation of pressing the solder layers with at leastone roller may be performed once or, if necessary, more than once. Theabove-described pressing may be performed on the entire surface of theprinted circuit board or only on a restricted area of the printedcircuit board selected according to the sizes and shapes of the printedcircuit board 2 and the roller 12, the dispositions of solder layers andmounted parts and convenience of operations. To press the entire surfaceof the printed circuit board once, the length of the roller 12 should belonger than the length of the printed circuit board. The solder layer 11may be formed by printing a cream solder according to another embodimentof the present invention.

FIG. 4 shows another embodiment of a method of flattening the uppersurfaces of solder layers 11 according to the present invention. In thisembodiment, a bonding tool 13 is used to connect the outer leads 7 ofthe TCP 3 and the lands 8 of the printed circuit board 2. The bottomsurface of the bonding tool 13 is pressed against the upper surfaces ofthe solder layers 11 to flatten the upper surfaces before the outerleads 7 are placed on the solder layers 11. In this manner, the uppersurfaces of the solder layers 11 can easily be flattened. According tothis embodiment, there is no need to prepare a pressing means such as aroller as shown in FIG. 3, and the desired flattening effect can beachieved by adding only one process step utilizing the bonding tool 13.

In still another embodiment of the flattening method of the presentinvention, the solder layers 11 are heated simultaneously with pressingusing the bonding tool 13. The solder layers 11 are thereby melted so asto be easily flattened. The bonding tool is cooled after melting thesolder layers 11, and is moved upward after the solder layers 11 havesolidified.

In a further embodiment of the flattening method of the presentinvention, a plurality of printed circuit boards are stacked one onanother and are pressed with a bonding tool, thereby flattening solderlayers of the plurality of printed circuit boards at one time.

In still another embodiment of the present invention, as shown in FIG.5, before a heat treatment, e.g., a reflow step where heat is applied tosolder layers 11, is performed on the printed circuit board 2 and lands8 and solder layers 11 are entirely covered with a heat resistant resinlayer 14 which can be easily removed afterward. As the heat resistantresin layer 14, an acrylic resin or UV curing type resin material may beused. The heat resistant resin layer 14 may be peeled off and removedfrom the surface of the printed circuit board after the heat treatmentand before TCP 3 is connected.

It is convenient to flatten the upper surfaces of solder layers 11 aspreviously described herein before covering with the heat resistantresin layer 14. For example, solder layers 11 can be flattened by beingpressed one or more times with a roller through their entire surfacesbefore parts are mounted on the printed circuit board 2, as in the casedescribed above with reference to FIGS. 3A and 3B. Also, only solderlayers which need to be flattened may be selectively pressed by aroller. In such a case, solder layers which do not need to be flattenedmay be previously covered with the heat resistant resin. If a printedcircuit board on which solder layers 11 are previously formed so as tohave flat surfaces is used, a heat resistant resin film can be coateddirectly thereon.

Solder layers are also formed on the lands 10 for connection of the subboard 4 and the lands for connection to the above-mentioned othersurface-mounted parts on the printed circuit board 2, as in the case ofthe lands 8 for connection of the TCP. These solder layers may be formedsimultaneously on the lands 8 by the above-described method when noparts are mounted on the printed circuit board 2. In a furtherembodiment of the present invention, the upper surfaces of solder layersother than those on the lands 8 can also be flattened before mountingthe sub board 4 and the above-mentioned other surface-mounted parts.

Flattening may be performed by rolling a roller such as roller 12 shownin FIG. 3B on the entire surface or a necessary part of the surface ofthe printed circuit board one or more times, as described above withrespect to the solder layers 11 on the lands 8. A pressing tool such asthe bonding tool 13 shown in FIG. 4 may also be used one or more timeson the entire surface or a necessary part of the surface of the printedcircuit board to flatten the solder layers. In this embodiment, it isconvenient to simultaneously flatten all the lands on the printedcircuit board 2 including the lands 8 for the TCP 3 and the lands 10 forthe sub board 4.

If the solder layer on the entire surface of the printed circuit boardis flattened simultaneously, the printed circuit board may be pinchedbetween flat plates each having a size larger than that of the printedcircuit board. The printed circuit board may be pressed so that appliedpressure is uniform through the entire surface of the printed circuitboard. As the flat plates, metallic plates, e.g., iron plates having astrength so as not to be deformed by a substantially large externalforce may be used. If the metallic plates have a size larger than thatof the printed circuit board, the solder layers on the entire surface ofthe printed circuit board can be simultaneously flattened, as mentionedabove. If flattening of only solder layers on a necessary portion of theprinted circuit board is desired, the plates may have a size smallerthan that of the printed circuit board. If it is difficult to press theentire surface of the printed circuit board simultaneously because ofthe existence of a protrusion or the like on the printed circuit board,the above-mentioned flat plates may be formed into any desired shape,for example, by being partially cut out. According to the presentinvention, it is possible to flatten all solder layers on a plurality ofprinted circuit boards by stacking the printed circuit boards andpressing the boards using the above-mentioned flat plates. In each ofthese cases, pressing and heating may be performed simultaneously.

As described above, the upper surfaces of the lands other than lands 8are flattened to enable the sub board 4 and the above-mentioned othersurface-mounted parts to be easily positioned and mounted with accuracyso as to be connected in a good condition. It is also possible to placeTCP 3 on the lands 8 simultaneously with these surface-mounted parts andto simultaneously solder TCP 3 and the other surface-mounted parts byreflow soldering, whereby the parts can be suitably connected withoutoccurrence of problems such as a lead misalignment, as in the case ofusing the bonding tool shown in FIG. 4. For the reflow step, any ofvarious well-known techniques, i.e., an overall heating method usinginfrared rays or hot air, local heating methods such as a laser methodor an optical beam method, and the like can be used. In a case where TCP3 cannot be mounted together with sub board 4 and other parts, forexample, due to the problem of the heat resistance of the carrier tape,it can be separately connected in a step after the step of mounting theabove-mentioned surface-mounted parts, as in the case of the embodimentdescribed above with reference to FIG. 2.

Further, on the printed circuit board 2 of the present invention, asshown in FIGS. 6 and 7, resin frames 15 and 16 are provided along thearrays of lands 8 corresponding to the arrays of outer leads 7 of TCP 3.The resin frames 15 and 16 are shaped approximately like four sides of asquare in this embodiment, so as to surround the arrays of lands 8 fromthe outside and inside of the same. Resin frames 15 and 16 are alsoformed in a similar manner even if the arrays of lands 8 form a shapeother than the above-mentioned square shape. For example, they may formfour sides of a rectangle in correspondence with the arrays of outerleads of TCP 3, or opposite or adjacent two of four sides of a square orrectangle. As shown in FIG. 7, each of the resin frames 15 and 16 isformed on a resist layer 17 formed on the surface of the printed circuitboard 2 so as to be sufficiently higher than the solder layer 11. Theresin frames 15 and 16 are formed of the same printing material ascharacters, symbols or the like provided on the printed circuit board 2by silk screen printing without increasing the number of process steps.Therefore, the resin frames 15 and 16 can be formed at a low costwithout requiring a considerably large amount of labor or time.

To connect TCP 3, flux 19 is first supplied to a position above thelands 8 and the solder layers 11 by using a well-known dispenser 18, asshown in FIG. 8A. Even if the viscosity of flux 19 is low, the flow offlux 19 is stopped by the resin frames 15 and 16 so that flux 19 isretained therebetween, thereby enabling flux 19 to be supplied to suchan amount that the solder layers 11 are entirely immersed in flux 19.The outer leads 7 are then placed on the solder layers 11 and are alsoat least partially immersed in the flux. When the outer leads 7 and thelands 8 are pressed and heated with the bonding tool 13 in this state,solder 20 sufficiently wets the outer leads 7 so as to suitably connectthe outer leads 7 and the lands 8, as shown in FIG. 8B.

In the above-described embodiment, resin frames 15 and 16 are formed onthe opposite sides of lands 8 so as to extend through a distancecorresponding to the entire length of each array of lands 8. However, ifa component other than the resin frames previously provided on theprinted circuit board can serve as a means for stopping the flow of fluxon one side of an array of lands 8, resin frames may be formed in such amanner that no resin frame portion is provided on this side of the arrayof lands 8 while a resin frame portion may be provided only on theopposite side. Also, if the flow of flux can be stopped by somecomponents locally on one or both sides of array of lands 8,corresponding resin frame portions may be removed.

According to the present invention, lands 8 corresponding to outer leads7 on all four sides of TCP 3 are connected simultaneously by performingpressing once and, accordingly, a four-side type bonding tool having agenerally-square bottom surface forming an open square space therein maybe used. In this embodiment, the four sides of the bottom surface of theabove-mentioned bonding tool are formed integrally. However, the bottomconfiguration of the bonding tool may alternatively be such that fourmembers formed separately from each other are combined to form a squareshape. Further, the portion of the bonding tool corresponding to eachside of a square shape may be formed of a plurality of members.

As shown in FIGS. 9 and 10, only lands 8 are formed in a squareframe-like area 21 defined on the printed circuit board 2 incorrespondence with the bottom surface configuration of the bonding tool13, and an upper surface 22 of the printed circuit board 2 is exposed inportions of the area 21 where no lands 8 are formed. In this manner,area 21, in which through holes, wiring patterns, a resist layer, andcharacters or the like printed on the resist layer by silk screenprinting are completely excluded from the arrays of lands 8 andextensions from the same, is provided on the printed circuit board incorrespondence with the bottom surface of the bonding tool 13 to ensurethat, as shown in FIG, 10, a desired parallelism is maintained betweenthe bottom surface 23 of the bonding tool 13 and the printed circuitboard 2 when outer leads 7 of the TCP are pressed by the bottom surface23 of bonding tool 13. Thus, a misalignment of leads at the time ofconnection is prevented even in the case of a package part such as a TCPhaving a particularly fine lead pitch, thereby reliably preventingoccurrence of connection failure. Wiring patterns or the like may beprovided within the area 21 as long as the height of the patterns is lowenough to avoid interference with the normal pressing operation of thebonding tool bottom surface 23. That is, the height of the patterns orthe like should be lower than the sum of the thicknesses of the lands 8and the leads 7.

As shown in FIG. 9, on the printed circuit board 2 of the presentinvention are also provided four marks 25 for positioning the TCP whenthe TCP is mounted. The four marks 25 are provided in an area 24indicated by the double dot-dash line. The area 24 is occupied by theCPU of the TCP when the TCP is mounted. In this embodiment, one mark 25is provided at each corner of the area 24 having a generally-squareshape. If the TCP is mounted as in the case of the present invention, itmay be necessary to allow the release of heat from the CPU and toconnect the CPU to a ground line. To do so, in the area 24, the CPU isbonded and fixed to the printed circuit board in a die bonding manner byusing a heat conductive resin, an electroconductive paste or the like.The double dot-dash line in FIG. 9 designates the outer periphery of diepads used for this die bonding or bonding portions.

While inwardly leading signal conductors from the lands and/or throughholes can be provided between the lands 8 and the area 24 on the printedcircuit board 2, no signal conductors are ordinarily provided in thearea 24 except for thermal vias connected to the die pads so as toconduct heat and to form ground line connections. Therefore, there is noproblem in terms of circuit design of the printed circuit board even ifpositioning marks 25 are provided in this area. Conversely, the degreeof freedom of circuit design can be increased by adopting thisarrangement. The positioning marks 25 are recognized at a stage beforethe step of setting the TCP 3 on the circuit board 2. Therefore, thereis no problem even if the marks are provided in the area where the CPUis fixed as in the case of the present invention.

Conventionally, such positioning marks are ordinarily provided in anarea outside the lands 8, as indicated by ghost-image marks 26 as shownin FIG. 9. Even in a case where such positioning marks are providedinside the lands 8, they are disposed at arbitrary positions on theprinted circuit board even in a signal conductor wiring area. Accordingto the present invention, a board area where no signal conductors arearranged is effectively utilized so that the overall size of asemiconductor device can be reduced while the packaging density andwiring density are increased, thus achieving an advantageous effect. Thepositioning marks 25 in accordance with the present invention can alsobe applied in the same manner to electronic parts other than TCPs ifthey are fixed by being bonded to a board.

Positioning marks 25 may be formed by patterning copper foil on theprinted circuit board as in the case of wiring patterns. Eachpositioning mark 25 and a region around the positioning mark 25 are notcovered with the resist. To improve the recognizability of positioningmarks 25, the order of lamination of the printed circuit board 2 may bedetermined while aligning the orientations of layers so that a CuO layeris formed by a blackening treatment 46 on an inner layer of 47 theprinted circuit board 2. As illustrated in FIG. 15 the inner layer, 47is adjacent to the outermost layer in this embodiment. Each positioningmark 25 is thereby made clearly recognizable by an optical sensor or thelike, thereby achieving correct and accurate mounting despiteminiaturization and high-density packaging of semiconductor devices.

FIG. 11 shows a further embodiment of the present invention in which anIC package to be mounted has leads provided only along a pair ofopposite sides of the IC package. In this case, an opposed pair of rowsof lands 27 are provided on a printed circuit board in correspondencewith the IC leads. In an area 28 defined by the lands 27, twopositioning marks 30 may be disposed at a diagonal pair of corners of anarea 29 corresponding to die pads or bonding portions for bonding andfixing an IC package chip when the IC package is mounted, as in the caseof the arrangement described above with reference to FIG. 9. Also inthis embodiment instead of two positioning marks 30, four positioningmarks can be respectively disposed at the corners, as in the arrangementshown in FIG. 9. In the embodiment shown in FIG. 9, of the fourpositioning marks 25, one of two diagonal pairs of the positioning marksmay be removed.

The preferred embodiments of the present invention have been describedin detail. However, as is apparent to those skilled in the art, thepresent invention comprises various changes and modifications of theabove-described embodiments within its technical scope. For example,cases of mounting a TCP have been described with respect to theembodiments but the present invention can also be applied in the samemanner to IC packages having a multiplicity of leads with a fine pitch.

The present invention is arranged as described above to have variousadvantages described below.

In the printed circuit board in the first aspect of the presentinvention, when an electronic part such as an SMD or TCP is mounted in asurface mounting manner by soldering, a misalignment is reliablyprevented between leads of the electronic part and lands to which theleads are to be connected, so that the leads and the lands can be stablymaintained in a good connected condition, thereby achieving animprovement in yield and a reduction in manufacturing cost. Inparticular, the circuit board can be designed so as to satisfy needs forincreasing the packaging density and the wiring density, for formingfiner patterns on the circuit board and for reducing the pitch betweenleads of parts in accordance with the recent development ofsemiconductor devices having highly improved functions.

The electronic parts connection method in the second aspect of theinvention ensures that leads of an electronic part can be correctlyaligned with solder layers on a printed circuit board even if the pitchof the leads of the electronic parts is reduced and even if the patternson the circuit board are formed more finely, and that the electronicpart can be mounted on the circuit board accurately and in a goodconnected condition by a certain kind of soldering such as reflowsoldering or local heating using a bonding tool. Moreover, theconventional mounting process is significantly improved with theaddition of few process steps. As a result, the productivity can beremarkably improved by practicing a simple method of the presentinvention at a low cost.

According to the third aspect of the present invention, frames areprovided on the printed circuit board to surround lands to enable asufficient amount of flux to be supplied to a region including thelands. Thus, leads can be sufficiently immersed in the flux so that thewettability of solder is improved. Therefore, even in the case ofconnecting a multiple-pin and narrow-pitch part such as a TCP, soldercan sufficiently wet leads so that the leads can be soldered in a goodconnected condition, thus achieving an improvement in yield.

In the case of the printed circuit board in the fifth aspect of thepresent invention, the desired parallelism of the bottom surface of abonding tool can always be maintained, so that there is substantially nopossibility of misalignment of leads. Thus, soldering can be suitablyperformed even in a case where an electronic part such as a TCP having asmall lead pitch is connected.

In the case of the printed circuit board in the sixth aspect of thepresent invention, positioning marks are provided in an area on acircuit board where die pads or the like for directly fixing anelectronic part chip for mounting are disposed. The circuit board areacan be effectively utilized without any hindrance to circuit design ofthe printed circuit board. Therefore, the degree of freedom of circuitboard design can be improved and the circuit board can be designed tofurther reduce the overall size while increasing the density.

According to the seventh aspect of the present invention, a multilayercircuit board is provided which has an inner layer processed by ablackening treatment in correspondence with the positions at whichpositioning marks are provided. The positioning marks can be clearlyrecognized by the effect of contrast with the inner layer processed bythe blackening treatment. It is therefore possible to improve theaccuracy with which electronic parts are mounted with the reduction inpattern size and the reduction in lead pitch of the circuit board.

What is claimed is:
 1. A printed circuit board comprising:a substrate onwhich at least one electronic part chip is mounted; and positioningmarks for positioning the at least one electronic part chip when thechip is mounted, wherein said substrate has a multilayer structurehaving an inner layer formed by a blackening treatment over which saidpositioning marks are located.
 2. A printed circuit board according toclaim 1, wherein the lands, a conductive layer inside the printedcircuit board, and the positioning marks are made of copper foil.
 3. Aprinted circuit board according to claim 2, wherein the inner layerformed by a blackening treatment is a CuO film.
 4. A printed circuitboard according to claim 1, wherein the inner layer formed by ablackening treatment is provided in the conductive layer directly underthe positioning marks.
 5. A printed circuit board comprising:a pluralityof lands for connection to leads of at least one electronic part, thelands being arranged in an array so that the lands point in a samedirection, solder being mounted on a top surface of the lands; arectangular region on said printed circuit board in which said array oflands are located including outermost ones of the lands, saidrectangular region including open regions extending from the outermostlands along longitudinal directions of the array of lands; a resistformed outside of the rectangular region; and a resin frame formed onthe resist outside of and surrounding the rectangular region, whereinonly said array of the lands are located on an upper surface of therectangular region of the printed circuit board, a height of the soldermounted on the top surface of the lands is a highest portion within therectangular region and a top surface of the resin frame is higher than atop surface of the solder.
 6. A printed circuit board according to claim5, wherein said at least one electronic part has leads arranged alongits four sides, and said rectangular region corresponds to said leadsarranged along one of said four sides.
 7. A printed circuit boardaccording to claim 5, wherein the rectangular region is formed so that abonding tool used for connection of the leads of the at least oneelectronic part to the lands can be positioned within the rectangularregion.
 8. A printed circuit board according to claim 7, wherein thepressing surface of the bonding tool has a rectangular shape, and theshape of the rectangular region on the printed circuit board is similarto the shape of the pressing surface of the bonding tool.
 9. A printedcircuit board comprising:a plurality of lands for connection to leads ofat least one electronic part, the lands being arranged in an array sothat the lands point in a same direction, solder being mounted on a topsurface of the lands; a rectangular region on the printed circuit boardin which the array of said lands are located including outermost ones ofthe lands, said rectangular region including open regions extending fromthe outermost lands along longitudinal directions of the array of lands;a resist formed outside of the rectangular region; and a resin frameformed on the resist outside of and surrounding the rectangular region,wherein only said array of lands are located on a surface of therectangular region, and when the leads of the at least one electronicpart are connected to the lands, a height of a top surface of the leadsbecomes a highest portion within the rectangular region and a topsurface of the resin frame is at a higher position than the lands.
 10. Aprinted circuit board according to claim 9, wherein the rectangularregion is formed so that a bonding tool used for connection of the leadsof the at least one electronic part to the lands can be positionedwithin the rectangular region.
 11. A printed circuit board according toclaim 10, wherein the pressing surface of the bonding tool has arectangular shape, and the shape of the rectangular region on theprinted circuit board is similar to the shape of the pressing surface ofthe bonding tool.